Ensure dates are properly formatted when overlapping years.
More context was needed than was provided to `ETH_Timeline::format_single_date()`, so determination of whether to show month and year moved to `ETH_Timeline::format_date()` as that method has the necessary context. As a result, `ETH_Timeline::format_single_date()` is now just a helper function for PHP's `date()` command. Fixes #7.
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